1. Field of the Invention
The present invention relates to an image signal correction method and an image signal correction apparatus, for suppressing changes in the hues of color images and for correcting gray levels during color image signal processing.
2. Related Background Art
Recently, for color images on television, an image signal correction process, such as gray-level correction or color correction, has frequently been employed to enhance or to correct ergonomic contrasts and ergonomic brightnesses or hues.
As a first conventional gray-level correction technique, processing is well known wherein gray-level correction having the concave input/output characteristics shown in FIG. 6 is performed for the color input luminance signal of an image to improve the ergonomic contrast of the image. For example, assume that luminance signals for three separate colors, red (R), green (G) and blue (B), are input. When the luminance signals input for the three colors RGB are (Rin1, Gin1, Bin1)=(0.3, 0.4, 0.5) and (Rin2, Gin2, Bin2)=(0.7, 0.8, 0.9), the gray-level transform shown in FIG. 6 is performed to obtain (Rout1, Gout1, Bout1)=(0.09, 0.16, 0.25) and (Rout2, Gout2, Bout2)=(0.49, 0.64, 0.81). When calculations are performed while the luminance level L of the image is defined as L=0.2125R+0.7154G+0.0721B (ITU-R BT709), Lin1=0.386, Lout1=0.152, Lin2=0.786 and Lout2=0.620 are obtained. The ratios of these levels are Lin2/Lin1=0.786/0.386=2.04, Lout2/Lout1=0.620/0.152=4.09, Lout1/Lin1=0.393 and Lout2/Lin2=0.789. In this case, the luminance is reduced but the contrast is increased.
As a second conventional gray-level correction technique, processing is well known whereby gray-level correction having the convex input/output characteristics shown in FIG. 7 is performed to improve the ergonomic brightness of an image. When luminance signals input for the three RGB colors are (Rin1, Gin1, Bin1)=(0.3, 0.4, 0.5) and (Rin2, Gin2, Bin2)=(0.7, 0.8, 0.9), the gray-level transform shown in FIG. 7 is performed to obtain (Rout1, Gout1, Bout1)=(0.548, 0.632, 0.707) and (Rout2, Gout2, Bout2)=(0.837, 0.894, 0.949). And through calculation, the luminance level L of the image is obtained as Lin1=0.386, Lout1=0.620, Lin2=0.786 and Lout2=0.886. The ratios of these levels are Lin2/Lin1=0.786/0.386=2.04, Lout2/Lout1=0.886/0.620=1.43, Lout1/Lin1=1.61 and Lout2/Lin2=1.13. In this case, the contrast is reduced but the luminance is increased.
However, a problem has arisen that these conventional examples have in common: the RGB ratio is also changed before and after the gray-level correction, and accordingly the hues are altered.
A technique to resolve this problem is disclosed in Japanese Patent Application Laid-Open No. H06-311354 (third conventional gray-level correction technique).
Employed for the technique disclosed in Japanese Patent Application Laid-Open No. H06-311354 is a configuration, shown in FIG. 8, comprising: a preliminary signal processing unit 21; a non-linear transforming unit 22; a color correction unit 23; a signal processing unit 24; a color image signal input terminal 25, for sequentially receiving color image signals; and a color image signal output terminal 26 for outputting color image signals. The preliminary signal processing unit 21 performs the preliminary image signal processing, such as the removal of noise, and the non-linear transforming unit 22 performs the gray-level correction. The color correction unit 23 performs the color correction for the signal output by the non-linear transforming unit 22, and the signal processing unit 24 performs the post-processing for the signal.
The non-linear transforming unit 22, which is the primary unit, calculatesM=max(Rin, Gin, Bin)Rout=f(M)·Rin/M Gout=f(M)·Gin/M Bout=f(M)·Bin/M  (Expression 1)and outputs the results. In Expression 1, max( ) denotes a function for selecting the maximum value; Rin, Gin and Bin denote input RGB color signals; Rout, Gout and Bout denote output RGB color signals; and f( ) denotes a gray-level correction function.
When Expression 1 is transformed, Expression 2 below is obtained,C=f(M)/M Rout=Rin·C Gout=Gin·C Bout=Bin·C  (Expression 2)
while the RGB ratio is not changed before and after the gray-level correction, and the hue is unchanged.
In Japanese Patent Application Laid-Open No. H06-311354, a configuration is also described wherein RGB signals are transformed into L*a*b*, or Luv, according to the CIE (Commission Internationale d'Eclairage), the gray-level correction is performed only for the luminance component (L* or L), and thereafter, the resultant signal is inversely transformed. Again, in this case, since only the luminance component is transformed and the color components are not changed, the hue is unchanged.
There is another conventional example wherein the gray-level correction is performed only for the Y component of a YUV signal, which is one type of television broadcasting signal.
As an additional conventional color correction example, there is one that employs well known processing whereby, as is shown in FIG. 9, the hue of an image is corrected by employing different gray-level correction functions for the color input luminance signals R, G and B, i.e., functions fr(x), for R, fg(x), for G, and fb(x), for B (wherein x is an arbitrary value). For example, when the input RGB luminance signals are (Rin1, Gin1, Bin1)=(0.3, 0.4, 0.5) and (Rin2, Gin2, Bin2)=(0.7, 0.8, 0.9), the gray-level correction shown in FIG. 9 is performed, and (Rout1, Gout1, Bout1)=(0.36, 0.4, 0.5) and (Rout2, Gout2, Bout2)=(0.84, 0.8, 0.9) are obtained. As a result, the ratio R obtained by the gray-level correction is greater than it was before the correction was made, and the reddish hue is enhanced in color.
Furthermore, Japanese Patent Application Laid-Open No. H08-315132 discloses, as a method for performing selective-corrections for an original image, a color correction method whereby, to change a selected, individual color, two or more selective color corrections are jointly employed in correspondence with the performance of a weighted, average correction process, during which a weighted value is reduced in consonance with selected changes in an original color.
There is also a case wherein, for an image, it is desired that, within a specific range, a color be enhanced without a hue being changed. And if the above described conventional color correction techniques, which use different gray-level correction functions for the RGB colors, were employed, the hue would be changed.
As a method for enhancing a color within a specific range without changing the hue, a third gray-level correction method described in related background art can be employed for the color enhancement. According to this method, different gray-level correction functions are provided for the RGB colors, gray-level correction is performed for the R, G or B color signal having the maximum value, and the other color signals are multiplied by the resultant signal, which uses as a coefficient the ratio of the RGB colors before and after the gray-level correction is performed (see FIG. 10).
The configuration in FIG. 10 comprises: a selector 31, for selecting the maximum value of the RGB signal values that are input; a comparator 32, for outputting information consonant with the R, G or B signal having the maximum value; a switch 33, for selecting, in accordance with the output of the comparator 32, either the R or the G or the B gray-level correction data, which will be described later; an R-gray-level correction data table 34 from which R-gray-level correction data are obtained in accordance with an instruction transmitted by the switch 33, a G-gray-level correction data table 35 from which G-gray-level correction data are obtained in accordance with an instruction transmitted by the switch 33 or a B-gray-level correction data table 36 from which B-gray-level correction data are obtained in accordance with an instruction transmitted by the switch 33, and a gray-level correction unit 37, for applying, for the output value of the selector 331, the gray-level correction data obtained from the gray-level correction data table 34, 35 or 36; a divider 38; and multipliers 39, 40 and 41.
When the RGB gray-level correction functions are as shown in FIG. 11 (the contents of the R-gray-level correction data table 34 are fr(x), indicated by a solid line, the contents of the G-gray-level correction data table 35 are fg(x), indicated by a broken line, and the contents of the B-gray-level correction data table 36 are fb(x), indicated by a chained line), and when (Rin, Gin, Bin)=(0.7, 0.3, 0.5), for example, is input, the comparator 32 determines that R has the highest value and transmits a corresponding signal to the switch 33, which selects the R-gray-level correction data table 34. From among the RGB values, the selector 31 selects a maximum value of 0.7 that it transmits to the gray-level correction unit 37 and the divider 38. The gray-level correction unit 37 corrects Rin by referring to the R-gray-level correction data table 34, and outputs a value 0.9 to the divider 38. The divider 38 divides the value 0.9, obtained following the correction, by the value 0.7, input before the correction, and outputs the value 1.286 as a correction coefficient. The multipliers 39, 40 and 41 multiply the original RGB values by the correction coefficient received from the divider 38, and output (Rout, Gout, Bout)=(0.9, 0.39, 0.64). At this time, Rout:Gout:Bout=0.9:0.39:0.64=0.7:0.3:0.5=Rin:Gin:Bin, and the hue is maintained unchanged.
When (Rin, Gin, Bin)=(0.5, 0.7, 0.3) is input, the same processing is performed and the G-gray-level correction data table 35 is selected because G has the highest value, and (Rout, Gout, Bout)=(0.32, 0.45, 0.19) is output. At this time, Rout:Gout:Bout=0.32:0.45:0.19=0.5:0.7:0.3=Rin:Gin:Bin, and the hue is maintained unchanged.
Similarly, when (Rin, Gin, Bin)=(0.3, 0.5, 0.7) is input, (Rout, Gout, Bout)=(0.3, 0.5, 0.7) is output, and the color and the luminance are unchanged.
That is, in this case, the luminance is increased for the hue (a reddish hue) when R has the highest value, the luminance is reduced for the hue (a greenish hue) when G has the highest value, and the luminance is unchanged for the hue (a bluish hue) when B has the highest value.
Generally, when the convex characteristics are provided for the gray-level correction function that corresponds to the R, G or B hue to be enhanced, the luminance of the pertinent hue is increased. While when the concave characteristics are provided for the gray-level correction function that corresponds to the hue that is not to be enhanced, the luminance of this hue is reduced. By employing this method, a color within a specific range can be enhanced.
However, with this configuration the following problem is encountered. FIG. 12 is a graph showing R and G signals along a specific horizontal line on a display device, and a correction coefficient output by the divider 38 and a luminance value on a display screen. When a signal B has a value of 0 across the entire area, as is shown in FIG. 12, the maximum value of the R component is to the left, and is reduced toward the right. At the position where R is at its maximum, a G signal has a value of 0. The value of this G component is increased toward the right and is at its maximum at a location whereat the R component has a value of 0. Thus, the R and G values are equal at the center, while to the left thereof, an area is represented wherein a hue becomes increasingly reddish, while to the right thereof, an area is represented wherein the hue becomes increasingly greenish.
Since the maximum value of the R component is in the left half in the graph, the correction coefficient is calculated by using the R-gray-level correction function fr(x) in FIG. 11. In this example, the correction coefficient is always one or greater, and in the graph, is increased to the right and upward. Further, since the maximum value of the G component is in the right half, the correction coefficient is calculated by using the G-gray-level correction function fg(x) The obtained correction coefficient is always one or smaller, and is reduced to the left and downward. The changes in the luminance at this time are as shown in FIG. 12, and the difference in the luminance levels appears as a border in the center of the graph.